1. Field of the Invention
The present invention relates to a multiple wavelength surface-emitting laser device and a method for its manufacture and, more particularly, to the multiple wavelength surface-emitting laser device that is equipped with a plurality of vertical cavity surface-emitting lasers, each of which emits light with a different wavelength, formed by a continuous process on a semiconductor substrate and a method for its manufacture.
This application claims priority form Korean Patent Application No. 2000-48986 which is incorporated herein by reference.
2. Description of the Related Art
In general, a surface-emitting laser emits light in a direction perpendicular to is its semiconductor material layers, unlike an edge emitting laser. This surface-emitting laser has a small radiating angle and emits a beam with a substantially circular cross-section unlike the edge emitting laser, and is easy to arrange in two dimensions, since the surface-emitting laser can be formed by integrating with a plurality of lasers in a desired array on a semiconductor wafer. Therefore, the surface-emitting laser has many advantages over the edge emitting laser in its application.
Commonly, in the field where a plurality of lasers with different wavelengths are needed, such as a LAN system using light and short and long distance communications using WDM (wavelength-division multiplexing), surface-emitting lasers corresponding to the desired wavelengths were manufactured separately and these were put together on a substrate for use. However, the method of putting together on a substrate a plurality of separately manufactured surface-emitting lasers results in a larger optical alignment error due to combining error between the surface-emitting lasers. Also the manufacturing process is complex and the manufacturing cost is thus high, and the above advantages of the surface-emitting laser have not been sufficiently utilized.
In order to fully utilize the advantages of the surface-emitting laser device, it has been suggested that a multiple wavelength surface-emitting laser structure be manufactured by adding a layer for controlling the resonant wavelength to a general surface-emitting laser structure and by a continuous manufacturing process.
Referring to FIG. 1, a conventional multiple wavelength surface-emitting laser has a semiconductor layer structure comprising a bottom-DBR (distributed bragg reflector) layer 1, an AlAs supplying layer 3, an AlGaAs tuning layer 5, an active layer 7, a high resistance part 9 that confines electric current, and a top-DBR layer 11. Here, the curved part of the AlGaAs tuning layer 5 shows the oxidated region, and the region of AlGaAs tuning layer 5 exposed to an atmosphere containing water vapor and oxidated from outside does not have a uniform thickness due to a thickness of the AlGaAs tuning laser 5. If the AlGaAs tuning layer 5 is thin, then the thickness of the oxidated region becomes relatively uniform.
In the conventional multiple wavelength surface-emitting laser device having the semiconductor layer structure as described above, the resonance length is changed by controlling the oxidation ratio of the AlGaAs tuning layer in a later process, in order to obtain the desired resonance wavelength. The change of the refractive index of the AlGaAs tuning layer from 3.4 to 1.6 corresponds to the reduction of its thickness by 12%.
Therefore, since it is possible to change the resonance wavelength by changing the oxidation ratio of the AlGaAs tuning layer through a later process as described above, a multiple wavelength surface-emitting laser device that emits light of a desired wavelength from each of a plurality of surface-emitting lasers that are manufactured in an array in a continuous manufacturing process, can be manufactured.
However, in the conventional multiple wavelength surface-emitting laser device as described above, the oxidation ratio changes rapidly depending on the Al content of the AlGaAs tuning layer 5, and thus precise process control is difficult and reproducibility is low. In addition, mechanical stress is generated between the AlGaAS tuning layer and AlAs supplying layer 3 during oxidation, thus the reliability of the laser device is low.
Furthermore, since the oxidated AlGaAs tuning layer 5 does not conduct electricity, the conventional multiple wavelength surface-emitting laser device with the structure described above should have a structure for electrical contact on top (the A region) of the AlGaAs tuning layer, as shown in FIG. 1, and therefore the structure is complex and requires many processes.
The present invention is contrived in consideration of the disadvantages described above, and it is thus an objective of the present invention to provide a multiple wavelength surface-emitting laser device of which the resonance wavelength can be controlled by an optical coating method using an optical deposition system, and the manufacturing process is simple and the reliability of the laser device is high compared to the prior art, and a method for its manufacture.
To achieve the above objective, a multiple wavelength surface-emitting laser device is equipped with a substrate and a plurality of surface-emitting lasers that are formed on the substrate by a continuous manufacturing process. Each surface-emitting laser comprises a bottom reflection layer on the substrate, that is doped with impurities of one type and that is composed of alternating semiconductor material layers having different refractive indexes; an active layer that is formed on the bottom reflection layer; an intermediate layer that is doped with impurities of the other type on the active layer; a top electrode that is formed on the intermediate layer to have a window through which light is emitted; and a dielectric reflection layer where dielectric materials with different refractive indexes are alternately layered on the intermediate layer and/or the top electrode to a thickness suitable for a desired resonance wavelength, which is controlled by adjusting the thickness of the dielectric reflection layer.
Here, the dielectric reflection layer is preferably composed of any two dielectric materials with different refractive indexes.
More preferably, the dielectric reflection layer is composed of two dielectric materials selected from the group consisting of TiO2, Ta2O5, ZrO2, HfO, SiO2 and MgF2.
The method for manufacturing the multiple wavelength surface-emitting laser device according to the present invention in order to achieve the above objective comprises a step of forming on a prepared substrate the bottom reflection layer that is doped with impurities of one type and that is alternately layered with semiconductor material layers having different refractive indexes, the active layer, and the intermediate layer that is doped with conductive impurities of a different type; a step of forming an arrangement of a plurality of surface-emitting lasers by removing the intermediate layer, the active layer and a part of the bottom reflection layer by etching; a step of forming on the intermediate layer of each surface-emitting laser a top electrode having a window through which light is emitted; and a step of forming on the intermediate layer and/or the top electrode of each surface-emitting laser a dielectric reflection layer where different dielectric materials are alternately layered to a thickness suitable for a desired resonance wavelength.
Here, the dielectric reflection layer is preferably formed by using an optical deposition unit.